A known image forming apparatus includes a belt unit. Generally, the belt unit is detachably attached to an apparatus body, e.g. a main body frame, of the image forming apparatus. The belt unit includes a positioning protrusion. The positioning protrusion is inserted into a positioning hole provided in the main body frame such that the belt unit is positioned relative to the main body frame.
To facilitate the operation for attaching the belt unit to the apparatus body, a diameter of the positioning hole can be greater than that of the positioning protrusion.
However, if the diameter of the positioning hole is greater than that of the positioning protrusion, when the positioning protrusion is only inserted into the positioning hole, a certain amount of play between the positioning hole and the positioning protrusion occurs due to a difference in diameter size, and the belt unit may not be accurately positioned relative to the main body.
When the positioning protrusion is inserted into the positioning hole, an outer surface of the positioning protrusion can be brought into contact with an inner wall surface defining the positioning hole, such that the play between the belt unit and the main body can be eliminated.
In the above known image forming apparatus, the positioning hole is formed like an elongated hole, and the positioning protrusion is formed like a cylindrical column. The cylindrical column is inserted into the elongated hole, the belt unit is urged downward by an urging force of a spring, and the belt unit is secured to the main body. With this structure, there is a high probability that the belt unit may be secured to the main body with the outer surface of the positioning protrusion spaced away from the inner wall of the positioning hole in a direction perpendicular to a direction of the urging force, especially in a direction of the minor axis of the positioning hole.
In short, there is a high probability that belt unit may be secured to the main body without accurate positioning.